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Developing and evaluating dose calculation models for verification of advanced radiotherapy
Umeå University, Faculty of Medicine, Department of Radiation Sciences.
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A prerequisite for modern radiotherapy is the ability to accurately determine the absorbed dose (D) that is given to the patient. The subject of this thesis has been to develop and evaluate efficient dose calculation models for high-energy photon beams delivered by linear accelerators. Even though the considered calculation models are general, the work has been focused on quality assurance (QA) tools used to independently verify the dose for individual treatment plans. The purpose of this verification is to guarantee patient safety and to improve the treatment outcome. Furthermore, a vital part of this work has been to explore the prospect of estimating the dose calculation uncertainties associated with individual treatment setups. A discussion on how such uncertainty estimations can facilitate improved clinical QA procedures by providing appropriate action levels has also been included within the scope of this thesis.

In order to enable efficient modelling of the physical phenomena that are involved in dose output calculations it is convenient to divide them into two main categories; the first one dealing with the radiation exiting the accelerator’s treatment head and a second one associated with the subsequent energy deposition processes. A multi-source model describing the distribution of energy fluence emitted from the treatment head per delivered monitor unit (MU) is presented and evaluated through comparisons with measurements in multiple photon beams and collimator settings. The calculations show close agreement with the extensive set of experimental data, generally within +/-1% of corresponding measurements.

The energy (dose) deposition in the irradiated object has been modelled through a photon pencil kernel solely based on a beam quality index (TPR20,10). This model was evaluated in a similar manner as the multi-source model at three different treatment depths. A separate study was focused on the specific difficulties associated with dose calculations in points located at a distance from the central beam axis. Despite the minimal input data required to characterize individual photon beams, the accuracy proved to be very good when comparing the calculated results with experimental data.

The evaluated calculation models were finally used to analyse how well the lateral dose distributions from typical megavoltage photon beams are optimized with respect to the resulting beam flatness characteristics. The results did not reveal any obvious reasons why different manufacturers should provide different lateral dose distributions. Furthermore, the performed lateral optimizations indicate that there is room for improved flatness performance for the investigated linear accelerators.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2006. , 50 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1044
Keyword [en]
Radiation therapy, high-energy photons, dose calculation, multi-source model, pencil kernel, uncertainties, action levels
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:umu:diva-841ISBN: 91-7264-141-X (print)OAI: oai:DiVA.org:umu-841DiVA: diva2:144711
Public defence
2006-09-22, 244, 7, Norrlands universitetssjukhus, Umeå, 13:00 (English)
Opponent
Supervisors
Available from: 2006-08-31 Created: 2006-08-31 Last updated: 2012-04-03Bibliographically approved
List of papers
1. A widely tested model for head scatter influence on photon beam output
Open this publication in new window or tab >>A widely tested model for head scatter influence on photon beam output
2003 (English)In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 67, no 2, 225-238 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: To construct and test a semi-analytical model describing the effects on Monitor Unit (MU) verification caused by scattering in the treatment head. The implementation of the model should be accomplished using a small set of experimental data. Furthermore, the model should include a geometry dependent estimation of the resulting uncertainty.

Material and methods: The input required by the created model consists of basic treatment head geometry and 10 measured output factors in air (OFair) for square fields. It considers primary energy fluence, scattered radiation from an extra-focal source and from secondary collimators, as well as backscatter to the monitor chamber. Measurements and calculations were performed in open symmetric and asymmetric fields at points located both on and off the collimator axis, as well as at arbitrary treatment distances. The model has been verified for 19 photon beams in the range from 4 up to 50 MV, provided by nine different treatment units from six manufacturers.

Results: The presented model provided results with errors smaller than 1% (2 S.D.) in typical clinical situations for all beams tested. In more exceptional situations, i.e. combinations of unconventional treatment head designs, very elongated fields, and dosimetry points far away from the isocenter, the total uncertainty increased to approximately 2%. The spread in the results was further analysed in order to create a method for predicting the uncertainties under different treatment conditions.

Conclusions: A general head scatter model that is easy to implement has been developed and can be used as the basis for computerised MU verification. The model handles all commercially available treatment units adequately and also includes an estimation of the resulting uncertainty.

National Category
Cancer and Oncology
Identifiers
urn:nbn:se:umu:diva-5253 (URN)10.1016/S0167-8140(02)00409-7 (DOI)12812855 (PubMedID)
Available from: 2006-08-31 Created: 2006-08-31 Last updated: 2017-12-14Bibliographically approved
2. Evaluation of uncertainty predictions and dose output for model-based dose calculations for megavoltage photon beams
Open this publication in new window or tab >>Evaluation of uncertainty predictions and dose output for model-based dose calculations for megavoltage photon beams
Show others...
2006 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 7, 2548-2556 p.Article in journal (Refereed) Published
Abstract [en]

In many radiotherapy clinics an independent verification of the number of monitor units (MU) used to deliver the prescribed dose to the target volume is performed prior to the treatment start. Traditionally this has been done by using methods mainly based on empirical factors which, at least to some extent, try to separate the influence from input parameters such as field size, depth, distance, etc. The growing complexity of modern treatment techniques does however make this approach increasingly difficult, both in terms of practical application and in terms of the reliability of the results. In the present work the performance of a model-based approach, describing the influence from different input parameters through actual modeling of the physical effects, has been investigated in detail. The investigated model is based on two components related to megavoltage photon beams; one describing the exiting energy fluence per delivered MU, and a second component describing the dose deposition through a pencil kernel algorithm solely based on a measured beam quality index. Together with the output calculations, the basis of a method aiming to predict the inherent calculation uncertainties in individual treatment setups has been developed. This has all emerged from the intention of creating a clinical dose/MU verification tool that requires an absolute minimum of commissioned input data. This evaluation was focused on irregular field shapes and performed through comparison with output factors measured at 5, 10, and 20 cm depth in ten multileaf collimated fields on four different linear accelerators with varying multileaf collimator designs. The measurements were performed both in air and in water and the results of the two components of the model were evaluated separately and combined. When compared with the corresponding measurements the resulting deviations in the calculated output factors were in most cases smaller than 1% and in all cases smaller than 1.7%. The distribution describing the calculation errors in the total dose output has a mean value of -0.04% and a standard deviation of 0.47%. In the dose calculations a previously developed correction of the pencil kernel was applied that managed to contract the error distribution considerably. A detailed analysis of the predicted uncertainties versus the observed deviations suggests that the predictions indeed can be used as a basis for creating action levels and tracking dose calculation errors in homogeneous media. (C) 2006 American Association of Physicists in Medicine.

Identifiers
urn:nbn:se:umu:diva-13388 (URN)10.1118/1.2207316 (DOI)16898459 (PubMedID)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2017-12-14Bibliographically approved
3. Dose uncertainties in photon pencil kernel calculations at off-axis positions
Open this publication in new window or tab >>Dose uncertainties in photon pencil kernel calculations at off-axis positions
2006 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 9, 3418-3425 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this study was to investigate the specific problems associated with photon dose calculations in points located at a distance from the central beam axis. These problems are related to laterally inhomogeneous energy fluence distributions and spectral variations causing a lateral shift in the beam quality, commonly referred to as off-axis softening (OAS). We have examined how the dose calculation accuracy is affected when enabling and disabling explicit modeling of these two effects. The calculations were performed using a pencil kernel dose calculation algorithm that facilitates modeling of OAS through laterally varying kernel properties. Together with a multisource model that provides the lateral energy fluence distribution this generates the total dose output, i.e., the dose per monitor unit, at an arbitrary point of interest. The dose calculation accuracy was evaluated through comparisons with 264 measured output factors acquired at 5, 10, and 20 cm depth in four different megavoltage photon beams. The measurements were performed up to 18 cm from the central beam axis, inside square fields of varying size and position. The results show that calculations including explicit modeling of OAS were considerably more accurate, up to 4%, than those ignoring the lateral beam quality shift. The deviations caused by simplified head scatter modeling were smaller, but near the field edges additional errors close to 1% occurred. When enabling full physics modeling in the dose calculations the deviations display a mean value of -0.1%, a standard deviation of 0.7%, and a maximum deviation of -2.2%. Finally, the results were analyzed in order to quantify and model the inherent uncertainties that are present when leaving the central beam axis. The off-axis uncertainty component showed to increase with both off-axis distance and depth, reaching 1% (1 standard deviation) at 20 cm depth. (c) 2006 American Association of Physicists in Medicine.

Keyword
dosimetry, physiological models, radiation therapy, dose calculation, uncertainties, off-axis, pencil kernel, head scatter
Identifiers
urn:nbn:se:umu:diva-13322 (URN)10.1118/1.2335488 (DOI)17022238 (PubMedID)
Available from: 2007-02-27 Created: 2007-02-27 Last updated: 2017-12-14Bibliographically approved
4. Optimization of photon beam flatness for radiation therapy
Open this publication in new window or tab >>Optimization of photon beam flatness for radiation therapy
2007 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 52, no 6, 1735-1746 p.Article in journal (Refereed) Published
Abstract [en]

In this work, we investigate the relation between lateral fluence/dose distributions and photon beam uniformity, possibly identifying ways to improve these characteristics. The calculations included treatment head scatter properties associated with three common types of linear accelerators in order to study their impact on the results. For 6 and 18 MV photon beams the lateral fluence distributions were optimized with respect to the resulting calculated flatness, as defined by the International Electrotechnical Commission (IEC), at 10 cm depth in six different field sizes. The limits proposed by IEC for maximum dose ratios ('horns') at the depth of dose maximum have also been accounted for in the optimization procedure. The conclusion was that typical head scatter variations among different types of linear accelerators have a very limited effect on the optimized results, which implies that the existing differences in measured off- axis dose distributions are related to non- equivalent optimization objectives. Finally, a comparison between the theoretically optimized lateral dose distributions and corresponding dose measurements for the three investigated accelerator types was performed. Although the measured data generally fall within the IECrequirements the optimized distributions show better results overall for the evaluated uniformity parameters, indicating that there is room for improved flatness performance in clinical photon beams.

Identifiers
urn:nbn:se:umu:diva-5256 (URN)10.1088/0031-9155/52/6/013 (DOI)
Available from: 2006-08-31 Created: 2006-08-31 Last updated: 2017-12-14Bibliographically approved

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